feat: add CocoaPods Podfile ecosystem checker

[algomaster99]

Summary

• Adds pkg/cocoapods, checking pod pins in Podfile against CocoaPods' trunk registry, via git-pkgs/manifests + an injected resolver.Resolver.
• Prioritized (see Add more ecosystems (Cargo, RubyGems, Composer, NuGet, CocoaPods, Bazel, Go, GitHub Actions) #4) because there's no CocoaPods equivalent of cargo add/go get/npm install: pod install/pod update only resolve versions for pods already listed in the Podfile — neither one adds a new pod line, so adding a dependency means hand-typing pod "Name", "x.y.z".
• CocoaPods reuses RubyGems' requirement operators; a bare version is an exact pin (requireOperator=false), but git-pkgs/vers has no dedicated "cocoapods" scheme, and its generic constraint parser doesn't recognize the ~> pessimistic operator as an operator at all — it would otherwise misparse "~> 1.0" as a literal exact version. isPessimisticConstraint filters that out before it reaches pins.ExactVersion.
• Wired into newCheckers/checkerFor in main.go, covered by unit tests, and added to benchmark/cases.json (4 fresh + 1 existing).

Before: no Podfile support — a stale pod version pin passes through unchecked.

After:

outdated dependencies, use these versions instead:
  Alamofire  5.4.0 -> 5.9.1

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